Re: [PATCH] iov_iter: don't require contiguous pages in iov_iter_extract_bvec_pages

Eric Dumazet
2024-11-01T17:05:53Z

On 10/24/24 7:00 AM, Christoph Hellwig wrote:
> From: Ming Lei <ming.lei@redhat.com>
>
> The iov_iter_extract_pages interface allows to return physically
> discontiguous pages, as long as all but the first and last page
> in the array are page aligned and page size. Rewrite
> iov_iter_extract_bvec_pages to take advantage of that instead of only
> returning ranges of physically contiguous pages.
>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> [hch: minor cleanups, new commit log]
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> lib/iov_iter.c | 67 +++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 45 insertions(+), 22 deletions(-)
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 1abb32c0da50..9fc06f5fb748 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1677,8 +1677,8 @@ static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
> }
>
> /*
> - * Extract a list of contiguous pages from an ITER_BVEC iterator. This does
> - * not get references on the pages, nor does it get a pin on them.
> + * Extract a list of virtually contiguous pages from an ITER_BVEC iterator.
> + * This does not get references on the pages, nor does it get a pin on them.
> */
> static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
> struct page ***pages, size_t maxsize,
> @@ -1686,35 +1686,58 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
> iov_iter_extraction_t extraction_flags,
> size_t *offset0)
> {
> - struct page **p, *page;
> - size_t skip = i->iov_offset, offset, size;
> - int k;
> + size_t skip = i->iov_offset, size = 0;
> + struct bvec_iter bi;
> + int k = 0;
>
> - for (;;) {
> - if (i->nr_segs == 0)
> - return 0;
> - size = min(maxsize, i->bvec->bv_len - skip);
> - if (size)
> - break;
> + if (i->nr_segs == 0)
> + return 0;
> +
> + if (i->iov_offset == i->bvec->bv_len) {
> i->iov_offset = 0;
> i->nr_segs--;
> i->bvec++;
> skip = 0;
> }
> + bi.bi_size = maxsize + skip;
> + bi.bi_bvec_done = skip;
> +
> + maxpages = want_pages_array(pages, maxsize, skip, maxpages);
> +
> + while (bi.bi_size && bi.bi_idx < i->nr_segs) {
> + struct bio_vec bv = bvec_iter_bvec(i->bvec, bi);
> +
> + /*
> + * The iov_iter_extract_pages interface only allows an offset
> + * into the first page. Break out of the loop if we see an
> + * offset into subsequent pages, the caller will have to call
> + * iov_iter_extract_pages again for the reminder.
> + */
> + if (k) {
> + if (bv.bv_offset)
> + break;
> + } else {
> + *offset0 = bv.bv_offset;
> + }
>
> - skip += i->bvec->bv_offset;
> - page = i->bvec->bv_page + skip / PAGE_SIZE;
> - offset = skip % PAGE_SIZE;
> - *offset0 = offset;
> + (*pages)[k++] = bv.bv_page;
> + size += bv.bv_len;
>
> - maxpages = want_pages_array(pages, size, offset, maxpages);
> - if (!maxpages)
> - return -ENOMEM;
> - p = *pages;
> - for (k = 0; k < maxpages; k++)
> - p[k] = page + k;
> + if (k >= maxpages)
> + break;
> +
> + /*
> + * We are done when the end of the bvec doesn't align to a page
> + * boundary as that would create a hole in the returned space.
> + * The caller will handle this with another call to
> + * iov_iter_extract_pages.
> + */
> + if (bv.bv_offset + bv.bv_len != PAGE_SIZE)
> + break;
> +
> + bvec_iter_advance_single(i->bvec, &bi, bv.bv_len);
> + }
>
> - size = min_t(size_t, size, maxpages * PAGE_SIZE - offset);
> iov_iter_advance(i, size);
> return size;
> }

This is causing major network regression in UDP sendfile, found by syzbot.

I will release the syzbot report and this fix :

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 65ec660c2960..e19aab1fccca 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1728,6 +1728,10 @@ static ssize_t iov_iter_extract_bvec_pages(struct
iov_iter *i,
(*pages)[k++] = bv.bv_page;
size += bv.bv_len;

+               if (size > maxsize) {
+                       size = maxsize;
+                       break;
+               }
if (k >= maxpages)
break;